Cemented carbide bodies, such as cutting members, conventionally are manufactured by powder pressing and sintering. Cemented carbides are composites comprising small grains in the micro meter range of at least one hard phase in a binder phase. The hard phase in cemented carbide is generally tungsten carbide (WC). In addition, other metal carbides with the general composition (Ti,Nb,Ta,W)C may also be included, as well as metal carbonitrides, for example, Ti(C,N). The binder phase usually comprises cobalt (Co) as a main constituent. Other compositions for the binder phase may also be used, for example, combinations of Co, Ni, and Fe, or Ni and Fe.
Industrial production of cemented carbide often includes mixing of given proportion of powders of raw materials and additives in the wet state using a milling liquid. The liquid is often an alcohol, for example, ethanol, or water, or a mixture thereof. The mixture is then milled into a homogeneous slurry. The wet milling operation is made with the purpose of deagglomerating and mixing the raw materials intimately. Individual raw material grains are also disintegrated to some extent. The obtained slurry is then dried and granulated, for example, by means of a spray dryer, and the granulate obtained may be used in uniaxial pressing of green bodies, in extrusion, or in injection moulding.
Hard metal bodies used for cutting members are conventionally manufactured by pressing and sintering. The desired form of the sintered body has to be obtained as far as possible before sintering because machining of a sintered body is expensive. Machining to the desired shape is therefore done, if necessary, in the as-pressed and/or pre-sintered condition after which the body is finally sintered. During sintering the body normally shrinks about 17% linearly
The term “pressing” used here and in the following concerns pressing a material powder, such as tungsten carbide (WC) together with cobalt (Co), between a punch and a die such that a green body is formed. The pressing may be uniaxial or multiaxial.
For the manufacturing of parts with complex geometry, injection moulding or extrusion is preferably used. Injection moulding is often referred to as Powder Injection Moulding (PIM) when used in powder technology. Powder Injection Moulding requires a mould having the specific shape of the part to be manufactured. In powder injection moulding of tungsten carbide based hard metal parts, the following four consecutive steps are applied:
1. Mixing of granulated cemented carbide powder with a binder system to form a feedstock.
2. Performing Injection moulding using the mixed feedstock. The material is heated to 100-240° C., forced into a cavity with the desired shape, cooled and then removed from the cavity.
3. Removing the binder system from the obtained part, also called debinding.
4. Sintering of the parts. Common sintering procedures for cemented carbides are usually applied.
During sintering of a powder injection moulded part the part normally shrinks about 20% linearly
Powder injection moulding of a large cemented carbide body sometimes results in cracks that are formed in the body during the sintering operation following the injection moulding. The problem with cracks after injection moulding, debinding and sintering tends to increase with increased material thickness of the injection moulded body.
U.S. Pat. No. 5,333,520 discloses a method for manufacturing a cemented carbide body for cutting tools, rock drilling tools or wear parts with complicated geometry. The cemented carbide body is manufactured by sintering together at least two parts that have been pressed to a green body before being sintered together. As the at least two parts shrink during sintering, there is a risk that the two parts will be subject to different shrinkage, and that this could lead to cracks in the cemented carbide body. Different shrinkage of the respective part could be a result of that different density is obtained in the part at the respective pressing operation.
It is therefore a desire to find an improved method to manufacture a metal cutting member of cemented carbide being composed of at least two parts formed in two separate forming operations without the risk for the formation of cracks in the cutting member.